ORI GIN AL PA PER

Porous rubber cryogels: effect of the gel preparationtemperature

Zeynep Oztoprak • Tugce Hekimoglu •

Ilknur Karakutuk • Deniz C. Tuncaboylu •

Oguz Okay

Received: 14 November 2013 / Revised: 26 March 2014 / Accepted: 15 April 2014 /

Published online: 27 April 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract This paper examines the effect of the gel preparation temperature (Tprep)

on the physical properties of the rubber-based macroporous organogels prepared by

solution crosslinking in benzene at subzero temperatures. Cis-polybutadiene (CBR)

and styrene-butadiene rubber (SBR) were used as the rubber components, while

sulfur monochloride (S2Cl2) was the crosslinker in the gel preparation. It was shown

that Tprep is an extremely important parameter to adjust the porous structure and

thus, the cryogel properties. The networks formed by CBR and SBR showed an

aligned porous structure with an exception of honey-comb structured porous SBR

cryogels prepared at -2 �C. 101- to 102-lm sized regular pores of the networks

caused by the benzene crystals act as a template during gelation, separated by

10–20 lm pore walls in thickness. They exhibit fast swelling and deswelling

properties as well as reversible swelling–deswelling cycles in toluene and methanol,

respectively. The ability of the organogels for the removal of petroleum products

from aqueous solutions was also demonstrated using diesel and crude oil as model

pollutants. In addition, the reusability of the organogels and their continuous

sorption capacities were checked by repeated sorption–squeezing cycles. All the

tests showed that the aligned porous organogels are suitable materials for the oil

spill cleanup procedures.

Keywords Macroporous � Aligned pore � Swelling � Gelation temperature �Cryogelation

Z. Oztoprak � T. Hekimoglu � I. Karakutuk � O. Okay

Department of Chemistry, Istanbul Technical University, 34469 Istanbul, Turkey

D. C. Tuncaboylu (&)

Faculty of Pharmacy, Bezmialem Vakif University, 34093 Istanbul, Turkey

e-mail: dtuncaboylu@bezmialem.edu.tr

123

Polym. Bull. (2014) 71:1983–1999

DOI 10.1007/s00289-014-1167-5

Introduction

Oil is one of the most important energy resources in the modern era. However, as

long as it is explored, transported and used there will be the risk of a spillage with

the potential to cause significant environmental damage. As a result of the huge

economic and environmental destruction from oil spills, several cleanup techniques

have been developed, such as the use of booms, skimmers, agricultural products,

dispersants and sorbents [1–5]. Among all the existing techniques, the use of

sorbents is of great interest because it is cheap, simple and effective. In addition, it

allows the collection and complete removal of oil. The efficiency of an oil sorbent

material is determined by properties, such as hydrophobicity, high uptake capacity,

retention over time, oil recovery, reusability or biodegradability.

In the previous reports, we have shown that frozen solutions of various types of

rubber including cis-polybutadiene (CBR), styrene-butadiene (SBR) and butyl

rubber (BR) can easily be crosslinked in organic solvents using sulfur monochloride

(S2Cl2) to produce tough and superfast responsive materials [6–9]. The rubber gels

thus obtained were macroporous and able to absorb large volumes of organic

solvents in a short period of time. It was also shown that porous rubber gels are

suitable sorbent materials for oil spill cleanup from surface waters due to their high

hydrophobicity, fast responsivity and reusability [6, 8].

Low-temperature gelation technique, known as cryogelation, was used to prepare

the organogel sorbents by crosslinking of rubber in organic solvents. This technique

is based on the natural principle that sea ice is less salty than sea water, i.e.,

crystallization results in the exclusion of solutes from growing crystals [10–19].

Similar with the nature, during the freezing of a rubber solution in benzene or in

cyclohexane, the solutes, i.e., the polymer chains and crosslinker molecules are

expelled from the solvent crystals and concentrates within the channels between the

crystals. Thus, the crosslinking reactions only take place in these unfrozen regions

acting as ‘‘microreactors’’. As demonstrated in Fig. 1, after the crosslinking

reactions and after thawing of the solvent crystals, a macroporous material is

produced whose microstructure is a negative replica of the crystals that formed. The

macropores are separated from each other by thick rubber network due to the

increased rubber concentration in the unfrozen part of the semi-frozen reaction

system. It is known that the size of the solvent crystals and thus the pores are

dependent on the freezing temperature which is not investigated before for the SBR-

and CBR-based cryogel systems.

In the present study, a series of CBR and SBR organogels were prepared at

various temperatures Tprep, which is the temperature of the thermostated bath in

which the reactions were carried out. The solution crosslinking reactions were

performed in benzene at a rubber concentration of 5 w/v%. The cryogels were

characterized by swelling tests as well as by scanning electron microscopy (SEM)

measurements on dry state. As will be shown below, the gel preparation temperature

Tprep is an important parameter to regulate the properties of cryogels such as

swelling behavior, porosity, sorption kinetics and morphology, and thus to obtain

aligned porous materials which have particular importance for application areas of

tissue engineering, microfluidics and organic electronics [20, 21].

1984 Polym. Bull. (2014) 71:1983–1999

123

Experimental

Materials

Cis-polybutadiene (CBR, Nizhnekamskneftekhim Inc.) and styrene-butadiene

rubber (SBR, Petroflex) were used as the rubber components. SBR is a copolymer

with 86 % internal unsaturated groups whereas each repeat unit of CBR has one

unsaturated group. The rubbers were dissolved in toluene followed by precipitation

in methanol and drying at room temperature under vacuum to a constant mass.

Weight average molecular weights �Mw of the rubbers were determined in

cyclohexane using a commercial multi-angle light scattering DAWN EOS (Wyatt

Technologies Corporation) equipped with a vertically polarized 30 mW Gallium-

arsenide laser operating at k = 690 nm and 18 simultaneously detected scattering

angles. The molecular weights, Mw, were 210 and 371 kg/mol for SBR and CBR,

respectively. The characteristics of SBR and CBR are shown in Table 1. The

crosslinking agent sulfur monochloride, S2Cl2, was purchased from Aldrich Co.

Fig. 1 Schematic representation of cryogel formation starting from reaction solution to a macroporouspolymer

Polym. Bull. (2014) 71:1983–1999 1985

123

Ta

ble

1C

har

acte

rist

ics

of

styre

ne-

buta

die

ne

and

CB

Rru

bber

s

Rubber

Code

Mole

cula

rw

eight

(g/m

ol)

Den

sity

(g/m

l)M

ole

cula

rfo

rmula

Sty

ren

e-b

uta

die

ne

rub

ber

(SB

R-1

50

2)

SB

R2

.19

10

50

.94

CH

2C

HC

H2

CH

CH

CH

2(

) n(

) m

Cis

-po

lyb

uta

die

ne

rub

ber

(CB

R-1

20

3)

CB

R3

.79

10

50

.93

(C

H2

CH

CH

CH

2) n

1986 Polym. Bull. (2014) 71:1983–1999

123

Benzene, toluene and methanol (all Merck reagents) were used as the solvents for

the solution crosslinking reactions, swelling and deswelling experiments, respec-

tively. Crude oil (d = 0.89 g/ml, viscosity = 350 cP at 25 �C) and diesel (setan

number 55, d = 0.85 g/ml, viscosity = 5.0 cP at 25 �C) used in the sorption

experiments were provided from Ozan Sungurlu wells of Turkey and BP ultimate

diesel, respectively.

Gelation reactions

The organogels were prepared by the solution crosslinking technique at a rubber

concentration, C0, of 5 % w/v according to the following scheme: rubber (5 g) was first

dissolved in 100 mL of benzene at 20 ± 1 �C overnight. Then, 25-mL portions of this

solution were transferred to volumetric flasks, and different amounts of sulfur

monochloride were added under rigorous stirring. The homogeneous reaction solutions

were transferred into plastic syringes of 16.4-mm internal diameters and glass Petri

dishes of 140 mm in diameter and 20 mm in height. The crosslinking reactions were

carried out in a cryostat at predetermined temperatures for 1 day. The crosslinker

concentration in the reaction solution was expressed as S2Cl2 v/w%, which is the volume

of S2Cl2 added per 100 g of rubber. Our previous works conducted in benzene at -18 �C

show that the reactions between S2Cl2 and the internal unsaturated groups of CBR or

SBR are complete, if S2Cl2 concentration is 6 v/w% [8]. In the present study, we fixed

the crosslinker content at 6 v/w% S2Cl2 as well as at half of this amount while the

gelation temperature was varied over a wide range. After the crosslinking, the reaction

system was thawed at room temperature for 1 h, and the formed gel was squeezed to

remove benzene. The gel was then washed several times first with toluene, then with

methanol, and finally, was dried under vacuum at room temperature.

Characterization

The gels were taken out of the syringes and they were cut into specimens of

approximately 10 mm in length. Each gel sample was placed in an excess of toluene

at 20 �C, and the toluene was replaced every other day over a period of at least

1 week to wash out the soluble polymer and the unreacted crosslinker. The swelling

equilibrium was tested by measuring the diameter of the gel samples through the use

of an image analyzing system consisting of a microscope (XSZ single zoom

microscope), a CDD digital camera (TK 1381 EG) and a PC with the data analyzing

system Image-Pro Plus. The swelling equilibrium was also tested by weighing the

gel samples. To dry the equilibrium swollen gel samples, the gels were first

immersed in methanol overnight and then dried under vacuum. The gel fraction,

Wg, defined as the amount of crosslinked (insoluble) polymer obtained from one

gram of rubber was calculated as follows:

Wg ¼mdry

10�2 mo C0

; ð1Þ

where mdry and mo are the weights of the gel samples after drying and just after

preparation, respectively, and C0 is the rubber concentration used in the gel

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preparation in w/v%. The equilibrium volume and weight swelling ratios of the gels

(qv and qw, respectively) were calculated as:

qv ¼V

Vdry

¼ D

Ddry

� �3

ð2Þ

qw ¼mswollen

mdry

; ð3Þ

where D and Ddry are the diameters of the equilibrium swollen and dry gels,

respectively, and mswollen is the weight of the equilibrium swollen gel. The swelling

measurements were conducted 6 times with an experimental error of 10–20 %.

For the texture determinations of the dried gels, scanning electron microscopy

(SEM) studies were carried out at various magnifications between 10 and 300 times

(Jeol JSM 6335F Field Emission SEM). Prior to the measurements, network

samples were sputter coated with gold for 3 min using a Sputter-coater S150 B

Edwards instrument.

Oil removal tests were conducted using organogel samples prepared in the form

of cylindrical tissues of 14 cm in diameter and 2 cm in thickness. All tests were

performed at 20 ± 1 �C. The kinetics of the oil sorption process were determined

by immersing 2 g of dry gel tissues into 500 mL of test solution and then

monitoring the mass of the gel as a function of time. The uptake capacity at selected

time intervals and the reusability as well as their continuous extraction capacities of

the sorbents for pollutants were determined by the methods described previously

[6]. This sorption–squeezing cycle was repeated 10 times to obtain the recycling

efficiencies and continuous extraction capacities of the rubber gels.

Results and discussion

SBR and CBR gels were prepared both at low temperatures, that is, below the

freezing point of benzene, and 25 �C, which were designated as cryogels and

conventional gels, respectively. Before characterization of the organogels, the

variation of the crosslinking efficiency of S2Cl2 depending on the gel preparation

temperature was investigated by the gel fraction measurements. It was found that

gel fraction of all rubber gels reported in this study was close to unity indicating

high crosslinking efficiency of S2Cl2 at temperatures down to -22 �C.

Effect of Tprep on swelling behavior

In Fig. 2, the swelling capacities of the rubber gels in toluene in terms of the

equilibrium weight (qw) and volume swelling ratios (qv) are plotted against the

gel preparation temperature Tprep. The gels were prepared starting from a 5 %

solution of SBR and CBR in benzene in the presence of 3 and 6 v/w% S2Cl2crosslinker.

At 3 % S2Cl2 (Fig. 2a, b), the weight swelling ratios qw of both SBR and CBR

gels formed at subzero temperatures are 45 ± 4, independent on the temperature

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Tprep between -22 and -2 �C. As expected, increasing S2Cl2 content from 3 to

6 %, decreases the swelling capacity of the organogels (Fig. 2c, d). Moreover, for

both CBR and SBR gels prepared below the freezing point of benzene, the

equilibrium weight swelling ratios qw are much larger than the equilibrium volume

swelling ratios qv. In contrast, the gels formed at 25 �C exhibit similar qw and qv

values. We should note that the weight swelling ratio qw includes the solvent located

in both pores and in the polymer region of the gel, while, assuming isotropic

swelling, the volume swelling qv only includes the solvent in the polymer region.

Therefore, the difference between values of qw and qv provide information about the

internal structure of gels such as their porosities in the swollen state [22]. The larger

Fig. 2 The equilibrium weight qw (open symbols) and volume swelling ratios qv (filled symbols) of SBR(a, c) and CBR (b, d) gels in toluene shown as a function of the gel preparation temperature Tprep.C0 = 5 w/v%, S2Cl2 = 3 (a, b) and 6 v/w% (c, d)

Polym. Bull. (2014) 71:1983–1999 1989

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the difference between qw and qv, the larger is the amount of solvent locating in the

pores, i.e., the larger is the total volume of pores.

Effect of Tprep on porosity

From the weight and volume swelling ratios of rubber gels, their swollen state

porosities (Ps) can be estimated using the equation [22]:

Ps % ¼ 1� qv

1þ ðqw � 1Þq=d1

� �102 ð4Þ

where q and d1 are the densities of the rubber and the swelling agent (toluene),

respectively. Assuming that q = 0.94 and 0.93 g/mL for CBR and SBR, respec-

tively (Table 1), and d1 = 0.876 g/mL, the calculated swollen state porosities Ps are

shown in Fig. 3a, b plotted against the Tprep for SBR and CBR gels, respectively.

The gels formed at subzero temperatures exhibit significant porosities. Ps is close to

90 at 3 % S2Cl2 contents while, for SBR gels, it slightly decreases to about 75 % as

the crosslinker content is increased to 6 %. Further, the swollen state porosity

rapidly decreases as Tprep is increased and becomes almost zero above the freezing

point of benzene.

Effect of Tprep on swelling–deswelling kinetics

CBR and SBR cryogels formed at various Tprep were subjected to swelling and

deswelling processes in toluene and in methanol, which are good and poor solvents,

respectively. For this purpose, the swollen gel was first immersed in methanol and

the weight change of the gel was determined as a function of the deswelling time.

After reaching the equilibrium state in methanol, the gel was immersed in toluene

and the reswelling process was monitored by recording the weight increase with

time. To check the durability of the gel sample against the volume changes, this

swelling–deswelling cycle was repeated twice.

Typical results for CBR gels are shown in Fig. 4 where the normalized gel mass

mrel (mass of gel at time t/equilibrium swollen mass in toluene) is plotted against the

time of deswelling in methanol (a, c) and reswelling in toluene (b, d). Completely

reversible swelling–deswelling cycles were obtained using gel samples prepared at

various temperatures. Independent of the Tprep, all the CBR gels attain their

equilibrium collapsed and equilibrium swollen states within 1 h. As will be seen in

the next section, all the gels formed at subzero temperatures have an interconnected

pore structure. This microstructure is responsible for their fast swelling and

deswelling rates. Thus, solvent molecules can enter or leave the cryogel through

interconnected pores by convection instead of the diffusion process that dominates

the conventional gels. They also exhibit completely reversible swelling–deswelling

cycles, i.e., the gels return to their original shape and mass after a short reswelling

period. Similar results were also obtained for organogel samples formed from frozen

SBR solutions.

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Fig. 3 Swollen state porosities Ps of SBR (a) and CBR (b) gels shown as a function of the gelpreparation temperature Tprep. C0 = 5 w/v%, S2Cl2 = 3 (open symbols) and 6 v/w% (filled symbols)

Fig. 4 Deswelling and reswelling cycles of CBR gels in methanol (a, c) and in toluene (b, d),respectively, shown as the variation of the relative gel mass mrel with the contact time. C0 = 5 w/v%,S2Cl2 = 6 v/w%. Tprep = -2 (filled circle), -6 (filled triangle), -10 (open triangle) and -22 �C (filledinverted triangle)

Polym. Bull. (2014) 71:1983–1999 1991

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Effect of Tprep on morphology

Macroporous morphology of rubber organogels was investigated after drying of the

gel samples under vacuum. Visual observations showed that all the cryogels retain

their original shape without any distortion in their cylindrical form during the drying

process, indicating the mechanical stability of their network structure. To visualize

the morphological texture and to explore the effect of the gel preparation

temperature on the size and especially the regularity of the pores, dried organogel

samples were examined by SEM. Figure 5 shows SEM images of SBR networks

formed at various Tprep between -22 and -2 �C. Initial rubber and the crosslinker

concentrations used in the gel preparation were 5 and 6 %, respectively. All the gel

samples formed below the freezing point of benzene (5.5 �C) have a regular porous

structure with pore sizes of 101–102 lm. We can explain the formation of regular

pores with the fact that benzene used as a solvent during the cryogelation reactions

is a good solvent for both CBR and SBR polymers over the whole range of Tprep [8].

Thus, due to the absence of a phase separation during the initial cooling period, only

cryogelation mechanism is responsible for the formation of the porous structures in

both SBR and CBR cryogels. After cryogelation, removing the frozen solvent that

acts as a porogen leads to a regular pore structure [14, 23, 24].

Fig. 5 SEM of SBR networks prepared at Tprep = -2, -6, -14, and -22 �C. C0 = 5 w/v%,S2Cl2 = 6 v/w%. The scaling bars and magnifications are 100 lm and 9100, respectively

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Figure 5 also shows that, as Tprep is decreased, the pore size decreases from

102–101 lm. Decreasing the gelation temperature means faster freezing of the

reaction solution which necessarily reduces the time available for solvent crystals to

grow. This would prevent the formation of large crystals. Since solvent crystals in

the cryogelation systems act as a template for the formation of pores, the same

relationship also exists between the pore size and Tprep of the cryogels. Under fast

freezing conditions, formation of small solvent crystals leads to the small porous

networks after thawing the reaction system. In addition, increasing freezing rate

shortens the duration of the initial nonisothermal period of the reactions so that the

crosslinking mainly occurs in the unfrozen microzones of the apparently frozen

reaction system which may also decrease the size of the pores [16]. Freezing point

depression may also be responsible for large pores of cryogels prepared at relatively

higher temperatures.

SEM images of the macroporous CBR networks formed at various Tprep are

shown in Fig. 6. Similar to the SBR networks, decreasing Tprep also decreases the

size of the pores and destroys their regularity due to the poor thermal conductivity

of the reaction solution. Thus, the pore size of SBR and CBR networks can be

reduced by decreasing the temperature of the cryogelation system. Note that

Fig. 6 SEM of CBR networks prepared at Tprep = -2, -10, -14 and -6 �C. C0 = 5 w/v%,S2Cl2 = 6 v/w%. The scaling bars are 100 lm (-2, -10 and -14 �C) and 10 lm (-6 �C)

Polym. Bull. (2014) 71:1983–1999 1993

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experiments carried out at smaller reactors with a diameter of 3.7 mm partially

destroyed the regularity of the pore structures due to the increasing rate of the

freezing of the reaction solution. Figure 6 (right-bottom) is also presenting the SEM

image of an organogel at a larger magnification indicating that the rubber cryogels

consist of an interconnected pore structure.

According to the theoretical results, the transition from gelation to cryogelation

regime occurs at temperatures close to the freezing point of the pure solvent [17].

For the CBR and SBR rubber system the transition occurs at around -2 �C, i.e.,

about 8 �C below the freezing point of benzene. When a polymer solution or an

aqueous suspension of solid particles is frozen very rapidly, the polymer or particles

are typically encapsulated into the freezing front. If the freezing rate is below a

Fig. 7 SEM of CBR networks prepared at Tprep = -10 and -14 �C. C0 = 5 w/v%, S2Cl2 = 6 v/w%.The scaling bars and magnifications are 1 mm and 910, respectively

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critical velocity, the polymer or solid particles are rejected from the freezing front

and concentrated between the orientated ice crystals [25]. As shown in Fig. 5, SBR

network has honey-comb morphology instead of aligned porous structures prepared

only at -2 �C which is the transition temperature of these systems. However, CBR

network has an aligned morphology at the same temperature (Fig. 6). The difference

between SBR and CBR cryogels related with the different molecular structure of

these polymers, i.e., bulky styrene groups on the main chain of SBR rubber that

affect critical velocity of the freezing system.

Freezing of the benzene starts from the surface of the cylindrical reactor which is

in contact with the cooling liquid and both the polymer chains and S2Cl2 are

encapsulated into the freezing front. By the time, across the moving freezing front

temperature and polymer concentration gradient establish. This leads to the

macroscopic instabilities due to which moving freezing front collapses, leaving

behind pockets of unfrozen concentrated polymer solution [9, 21]. After the

crosslinking reactions, the polymer phase becomes insoluble and, after thawing,

preserves this aligned structure. The micrographs in Fig. 7 evidence the alignment

of the pores in the CBR networks formed at -10 and -14 �C, respectively. The

images clearly indicate directional freezing of the solvent crystals in the direction

from the surface to the interior. In contrast to the regular and aligned morphology of

CBR and SBR networks prepared in benzene, the networks formed from butyl

rubber (BR) exhibited a broad size distribution of pores from micrometer to

millimeter size due to the poor solvent characteristic of benzene for BR leading to

the thermally induced phase separation mechanism [9].

Effect of Tprep on oil sorption capacity

Sorption properties of organogels were analyzed by immersing dry gel samples in

oil media until attaining swelling equilibria. Figure 8a shows the sorption kinetics

Fig. 8 Crude oil (triangles) and diesel (circle) sorption capacities of CBR gels prepared at Tprep = -18(open symbols) and -6 �C (filled symbols) shown as functions of the contact time (a) and the number ofcycles (b). C0 = 5 w/v%, S2Cl2 = 6 v/w%

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of the cryogels prepared at two different temperatures Tprep, namely at -6 and

-18 �C. Our previous work demonstrates that butyl rubber sorbent prepared at

-18 �C (-18BR) is an efficient material for oil spill cleanup [6]. Therefore, in the

present study, we selected -18 �C as well as -6 �C arbitrarily to compare and

demonstrate the effect of the temperature on the oil sorption capacities of the rubber

cryogels. Here, the sorbed amount of the model pollutants, diesel and crude oil, is

plotted against the contact time. It is seen that CBR cryogels absorb the pollutants in

\1 min to attain the thermodynamic equilibrium. The fast sorption rate is due to the

convection of the pollutants through the micrometer-sized and interconnected pore

structure (Fig. 6, right-bottom).

Figure 9 was derived from Fig. 8a to compare the maximum sorption capacities

of CBR gels for crude oil and diesel as pollutants. The gels prepared both at -18

and -6 �C have a higher sorption capacity for crude oil than the diesel. This is due

to the higher viscosity of crude oil that increases the adherence of oil onto the

surface of the rubber by favorable hydrophobic interactions between the crude oil

and the hydrophobic polymer. Figure 9 also shows that, decreasing the preparation

temperature from -6 to -18 �C, leads to a lower sorption capacity for the model

pollutants. For example, diesel sorption capacity of cryogels prepared at -6 �C is

17 g g-1, which is about 2 times of the cryogels formed at -18 �C. The same

tendency is observed for crude oil sorption capacities that are 18 and 22 g g-1 for

the gels prepared at -6 and -18 �C. We have to mention that widely used oil

sorbents based on polypropylene (PP) have sorption capacities 15 and 11 g g-1 for

crude and diesel, respectively. Thus, considering the gel preparation temperature

CBR cryogels has higher sorption capacities than commercial oil sorbent PP.

Fig. 9 Sorption capacities of CBR gels prepared at Tprep = -18 and -6 �C shown as functions of thetype of pollutants and Tprep. C0 = 5 w/v%, S2Cl2 = 6 v/w%

1996 Polym. Bull. (2014) 71:1983–1999

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Although the reusability of the sorbents is an important aspect of the oil spill

cleanup, commercial oil sorbents are not reusable and thus, they form a solid waste

after the cleanup procedure [3]. Therefore, the reusability of the present organogels

and their continuous sorption capacities were checked by repeated sorption–

squeezing cycles. The results of the sorption–squeezing cycles are shown in Fig. 8b,

where the sorption capacity of the cryogels in each cycle is plotted against the

number of cycles. Comparison of Fig. 8a, b shows that the amount of the sorbed

pollutant in each cycle is almost constant and is close to the maximum sorption

capacity, indicating the high reusability of the rubber cryogels.

Figure 10 compares the pollutant sorption capacities of CBR and SBR cryogels

prepared at -18 �C. CBR cryogels have a higher crude oil sorption capacity as

compared to SBR cryogels. In contrast, the sorption capacity of SBR cryogels for

diesel is larger than CBR cryogels. Therefore, there is no correlation between the

type of the rubber and their sorption capacities.

Mechanical properties of CBR and SBR rubber gels were also investigated by the

compression tests. The general trend is that all the gels prepared at subzero

temperatures between -2 and -22 �C exhibited moduli of elasticity around 2 kPa.

No substantial variation of the elastic modulus was observed depending on Tprep.

Moreover, all the low-temperature gels were very tough and can be compressed up

to about 100 % strain without any crack development.

Conclusions

Aligned porous rubber cryogels were prepared from frozen solutions of CBR and

SBR in benzene at various Tprep with an exception of honey-comb porous structured

Fig. 10 Sorption capacities of CBR (cyan) and SBR (pink) gels prepared at -18 �C shown as functionsof the type of pollutants and Tprep. C0 = 5 w/v%, S2Cl2 = 6 v/w% (color figure online)

Polym. Bull. (2014) 71:1983–1999 1997

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SBR cryogels prepared at -2 �C. The networks formed from CBR and SBR showed

101- to 102-lm sized regular pores caused by the benzene crystals acting as a

template during gelation. The pore size of the rubber networks can be reduced by

decreasing the gel preparation temperature of the cryogelation system. Cryogels

exhibited fast swelling and deswelling properties as well as reversible swelling–

deswelling cycles in toluene and methanol, respectively. The sorption tests showed

that CBR and SBR cryogels are efficient reusable sorbent materials for oil spill

cleanup due to their high sorption capacities for diesel and crude oil as well as their

reusability after simple squeezing.

Acknowledgments This work was supported by the Scientific and Technical Research Council of

Turkey (TUBITAK), TBAG–107Y178. O.O thanks the Turkish Academy of Sciences (TUBA) for their

partial support.

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